lead() on grouped data is buggy

[alex23lemm]

This is a follow up on #925 which fixed lag() on grouped data.

The same issue exists also for lead() when data is grouped and wasn't fixed yet in the current dev version:

df <- data.frame(name=rep(c('Al','Jen'),3),
                score=rep(c(100, 80, 60),2))
 df %>%
   group_by(name) %>%
   mutate(next.score = lead(score),
          before.score = lag(score) )
#   name score next.score before.score
#1   Al   100         80           NA
#2  Jen    80         60           NA
#3   Al    60        100          100
#4  Jen   100         80           80
#5   Al    80         NA           60
#6  Jen    60         NA          100

For instance, next.score for Al in line 1 should be 60 and not 80.

Following the fix of #925 I assume that Lead.h in line 52 needs to be modified from out[out_index[i]] = data[index[i]+n] ; to out[out_index[i]] = data[index[i+n]] ;

[romainfrancois]

Thanks. That was indeed the right fix. Given that you found it, perhaps you could have submitted a pull request ?

[alex23lemm]

Thanks for the feedback and the fix! You're right, I should have submitted a pull request. I doubt that there will be a second chance given the excellent state of the package :-).

Cheers,
Alex